Ducted wind turbine

ABSTRACT

The performance of a turbine in a duct having a convergent and a divergent surface is enhanced by controlling the fluid flow pattern along the inner duct surface. For this purpose a free rotor redistributes part of the inner fluid stream through a ring into the outer stream to prevent premature separation of the inner stream from the divergent duct surface. The turbine and rotor are driven by distinct fluid streams that are separated by a duct.

TECHNICAL FIELD OF INVENTION

The present invention is related to a duct and free rotor combinationused in energy conversion systems having means for extracting energyfrom a fluid stream. The invention is particularly applicable toCombined Augmentation Technology Turbines (CATT) for wind and underwaterapplications.

BACKGROUND OF THE INVENTION

There is global interest in the development of alternative energysources especially wind power.

The electrical power generated by such wind machines is oftentransmitted considerable distances to centres of population, and onereason for this is that people usually live in sheltered terrain wherethe wind resource is more modest. The less concentrated energy in thewind in such areas reduces the economics of energy capture withconventional propeller wind turbines.

It has been proposed to develop devices that concentrate wind energy byincreasing wind velocity, thus enabling wind with a lower velocity andenergy density to be used where it is available. These proposals canpotentially eliminate long power transmission lines.

One approach is a diffuser duct, which increases in cross-sectional areadownstream of the propellor. This duct creates a lower pressure, thusdrawing a faster air stream through the smaller diameter air inlet.Problems with diffusers typically include the control of turbulence whenthe airflow breaks away from the inner surface of the duct. Thisturbulence restricts flow and substantially reduces the benefit of adiffuser as wind velocity increases. U.S. Pat. No. 4,075,500 and U.S.Pat. No. 4,422,820 describe ducts which propose control of suchturbulence by introducing external airflows through a series oforifices.

A similar arrangement is shown in U.S. Pat. No. 4,132,499, whichincludes a separate annular aerofoil. Other proposals are found inWO-A-00/50769 and WO-A-01/06122.

Another way of concentrating wind energy is described in U.S. Pat. No.2,485,543 and U.S. Pat. No. 2,784,556. In this case a wind machineoperates on the depression principle and has a wind driven propellerwith hollow blades and apertures at the tips. Rotation of the propellerinduces air in the hollow blades to flow out through the apertures inthe blade tips. This in turn causes a lower pressure within the device,and by communicating through an air passage a faster stream of externalair is drawn through the system.

An aim of this invention is to improve energy conversion of aconventional turbine by increasing the velocity of the local fluidstream.

STATEMENT OF THE INVENTION

According to the invention there is provided a duct assembly comprisingan axial flow free rotor and a circular duct on the upstream sidethereof, said duct being adapted to substantially separate a fluidstream into an inner stream within the duct and an outer stream outsidethe duct, said rotor having a hub with a plurality of arms extendingradially outwardly from said hub, wherein each of said arms comprises aradially outer portion and a radially inner portion, the outer portionsbeing responsive to said outer stream to cause the free rotor to rotateunidirectionally in use, and the inner portions defining extractionmeans adapted to draw fluid from said inner stream towards the freerotor from the upstream side thereof.

The present invention provides a free rotor adapted to be driven by theouter stream, the free rotor being situated in use downstream of anenergy generation propeller or turbine and extracting a part of theinner stream that has flowed through the area described by this turbine.The inner stream is redistributed into the outer stream downstream ofthe free rotor.

In the specification the term ‘rotor’ is intended to genericallydescribe any suitable fluid driven device, such as a propellor orturbine, whereby fluid stream energy is converted into rotation of amechanical element.

When the fluid stream is flowing, the kinetic energy of the fluid actson the free rotor blades to induce the rotation of the apparatus, andmeans are provided by which centrifugal and mechanical forces act on theinner stream, and cause part of it to flow radially outwardly to mixwith the outer stream. By adapting the number and relative dimensions ofrotor elements and components, the extraction rate of the inner streamand pattern across the swept area of the free rotor can be altered.

In accordance with one specific aspect of a preferred embodiment hollowrotor blades preferably extend outwards from the diameter of the ductand are driven by an outer fluid stream flowing over the outside surfaceof the duct. The rotor blades are preferably connected to a bearing hubby solid spars, and the apparatus is free to rotate. A preferred featureis a ring-like channel of a similar dimension to the diameter of theduct, positioned between the hollow rotor blades and spars. Duringrotation a part of the inner fluid stream is extracted by means of theinner surface of the circular ring shaped channels and inlets to hollowrotor blades. Advantageously the confined flow pattern through the ductcan be controlled by adapting the free rotor in such a way as to varythe redistribution of the fluid and provide boundary layer control as ameans of preventing premature fluid separation along the inner surfaceof the duct. Through passages of the hollow rotor blades direct thefluid through respective expel apertures into the outer stream movingdownstream of the rotor. The rate of rotation of the free rotor has arelationship with the extraction rate and velocity of the primary fluidentering the duct, which can be determined empirically.

BRIEF DESCRIPTION OF DRAWINGS

Other features of the invention will be apparent from the description ofseveral preferred embodiments shown by way of example only in theaccompanying drawings in which:

FIG. 1 is a cross-section of a duct and free rotor assembly according toa first embodiment of the invention.

FIG. 2 is a cross-section of a second embodiment of the duct and freerotor assembly.

FIG. 3 is a-cross-section of a third embodiment including external andinternal ducts.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a mast 5 on which is mounted a circularconvergent/divergent duct 1 of symmetrical form and having a convergentsurface 6 and a divergent surface 8. Co-axially mounted in the duct 1 byone or more supports 4 is a centre body 2 within which is located arotary generator (not shown). A drive rotor 3, typically a propellor, isconnected to the generator and is rotatable about the axis thereof inresponse to an air stream represented by arrows W. The generator is ofany suitable kind, typically converting mechanical energy of thepropellor into electrical energy.

The duct 1 and/or mast 5 are pivotable so that the inlet 6 can bedirected into the air stream, and means such as a rudder may effectautomatic directional control. Alternatively the free rotor itself maybe arranged to give a rudder effect. The centre body 2 and/or duct 1 isrelatively pivotable in the preferred embodiment to permit feathering ofthe drive rotor 3 by turning out of the airstream.

As is well understood a convergent/divergent nozzle has a maximum flowvelocity at the throat 7, and accordingly the drive rotor 3 is alsoarranged at or adjacent the throat in accordance with known principlesof venturi design.

A free rotor 9 is provided at the downstream side of the centre body 2and is mounted for free rotation on the axis of the drive rotor 3. Thefree rotor 9 consists of a plurality of radial arms 13 which terminatein blades 12. The arms 13 can be equally spaced and can be symmetricaland of equal length. As illustrated the free rotor is immediatelyadjacent the outer of the duct 1, and the blades 12 are wholly radiallyoutside the periphery of the duct. The blades 12 can be of any suitableform, and are not shown in detail in the drawing.

The gap 11 between the free rotor 9 and the duct 1 should generally beas small as possible, depending on aerodynamic considerations.Furthermore in this embodiment the free rotor 9 has a generally axiallyextending ring 10 which extends on the leading and trailing side toassist extraction from the duct and to reduce turbulence. Otheraerodynamic features may be added to stabilize flow through the duct soas to enhance the efficiency of energy conversion, and to improve theefficiency of air extraction.

The leading portion of the ring 10 directs fluid radially outwardlythrough inlets 15 to hollow blades 12, and then to respective exhaustopenings 14 at the tips thereof. The corresponding inlet openings (notshown in detail) are provided in the arms 13 in the region of the outletof the duct 1.

In operation, the duct 1 is generally faced directly into the air streamW, and as a consequence the drive rotor 3 rotates with resultingconversion of energy at the generator. Primary air W_(p) flows over theouter surface of the duct 1 and acts on the blades 12 to cause the freerotor 9 to spin. This primary air W_(p) has not given up energy in thedrive rotor 3. Rotation of the free rotor causes a radially outward flowof air due to the ring 10. As a consequence the pressure of airimmediately upstream of the free rotor 9 is reduced, and accordinglysecondary air W_(s) from the drive rotor 3 is sucked towards the freerotor 9. This in turn increases the velocity of air at the throat 7 ofthe duct 1 and thus the speed of the drive rotor 3. In this way energyconversion by the drive rotor 3 can be enhanced.

It will be appreciated that the proportions of the duct, the kind ofdrive rotor, the size and design of arms and many other variablefeatures are matters of routine design and can be determined empiricallyor by calculation to suit circumstances at the installation site. In thesame way aerodynamic features of any conventional kind may be added toimprove energy conversion.

An alternative embodiment is illustrated in FIG. 2, and is particularlyadapted for use in flowing water. The arrangement is generally asillustrated in FIG. 1, the free rotor 16 having solid spars 19 andprimary drive blades 17. The spars 19 are designed to give the minimumresistance to rotation commensurate with adequate mechanical integrityof the free rotor 16. At the radially inner side of the drive blades 17,a circular array of secondary blades 18 are arranged to propel waterfrom the duct to the downstream side. The duct incorporates stators ordeflectors 21 in an annular chamber 20 to direct water to the secondaryblades 18 in a generally ‘S’ shaped path.

It will be appreciated that the principle of operation of thisembodiment is the same as that of FIG. 1, in that energy provided byprimary fluid to the blades 17 is used to draw secondary fluid throughthe duct in a manner that produces a controlled flow across the innersurface, and thereby increase the velocity of water flowing over thedrive rotor.

More than one free rotor may be provided in order to maximise energyrecovery from the fluid stream. In one embodiment a plurality of freerotors are provided in series and having a common axis of rotation.Contra-rotating free rotors may be provided to counter torsionaleffects.

The device according to the invention may also include means to drivethe free rotor at a minimum speed so as to ensure relatively smoothrunning in very light winds. It is envisaged that a small electric motormay be provided for this purpose, and that the free rotor may haveconsiderable mass in order not to be strongly influenced by variationsin the speed of the fluid stream.

In the case that exhaust apertures are provided at the tip of the freerotor, these apertures may be directed to assist rotation or to preventor reduce rotation by means of a direction device such as a flap valve.This arrangement ensures against overspeeding in high winds. Otherconventional speed control means may be included, including flexiblevane elements and the like.

The duct may be defined by a mesh or the like in the region immediatelyupstream of the free rotor, particularly in the case of the embodimentshaving stator elements in the duct. Such an arrangement may beadvantageous in controlling boundary layer effects.

In an alternative embodiment illustrated in FIG. 3 the duct mayincorporate one or more internal ducts 24 so as to provide a nestedarray of generally coaxial surfaces which diverge towards the freerotor. The internal ducts define annular volumes upstream of the freerotor. Such an array can improve flow control between the inner surfaceof the outermost duct, and the centre body 2. The internal ducts performthe function of guiding the fluid stream. As the fluid stream passesover the internal ducts a lower pressure is created inside the internalducts which further aids in sucking secondary air W_(s) from the driverotor 3 towards the free rotor 9.

The or each duct may also have a varying wall thickness, as illustratedfor example in the Figures to confer aerodynamic benefits and inparticular to give a substantially straight line path for primary airflowing over the outer duct surface.

1. A duct assembly comprising a circular duct and an axial flow freerotor on a downstream side thereof; said duct being open at both endsand adapted to substantially separate a unidirectional fluid stream intoan inner partial stream within the duct and an outer partial streamoutside the duct; said duct having an inner surface comprising aconvergent surface and a divergent surface; said free rotor having a hubwith a plurality of arms extending radially outwardly from said hubwherein each of said arms comprises a radially outer portion and aradially inner portion; the radially inner portions terminating at aring having through passages connecting the inner stream to the outerstream; said ring having a radially inner surface at an upstream edgethereof, adjacent the radially inner surface of said duct at thedownstream edge thereof; and the radially outer portions extending fromsaid ring and comprising blades responsive to said outer partial streamto cause the free rotor to rotate unidirectionally in use; and whereinsaid free rotor includes extraction means adapted to draw fluid fromsaid inner partial stream along the divergent surface of the ducttowards the free rotor from the upstream side thereof, and to cause apart of said inner partial stream to flow radially outwards via saidring to mix with said outer partial stream moving downstream of the freerotor; the fluid drawn through the through passages of the ring from theinner stream to the outer stream causing a redistribution of flowthrough the duct so as to provide boundary layer control to preventpremature separation of fluid along the inner surface of the duct.
 2. Aduct assembly according to claim 1 wherein said arms are equally spaced.3. A duct assembly according to claim 1 wherein said arms aresymmetrical and of equal length.
 4. A duct assembly according to claim 1and wherein said free rotor further includes blades adapted to draw saidpart of the inner partial stream radially outwardly from the upstream tothe downstream side via said ring.
 5. A duct assembly according to claim1 wherein said blades are hollow, and each define a through passage fromthe radially inner portion to the radially outer tip thereof.
 6. A ductassembly according to claim 1 wherein upstream of said free rotor, theduct diverges in the flow direction of said unidirectional fluid streamtowards said free rotor.
 7. A duct assembly according to claim 1 andincluding a generally coaxial inner duct which diverges in the directionof the fluid stream towards said free rotor, the ducts together definingan annular volume upstream of said free rotor.
 8. A duct assemblyaccording to claim 1 and further comprising a drive rotor upstream ofsaid free rotor and having a common axis of rotation, the drive rotorbeing responsive to the inner partial stream to cause unidirectionalrotation thereof, and the drive rotor being adapted for connection to agenerator.
 9. A duct assembly according to claim 8 wherein said driverotor is a propellor.
 10. A duct assembly according to claim 9 andmounted on a stand for pivoting in a horizontal plane.
 11. A ductassembly according to claim 8 wherein said duct diverges upstream ofsaid drive rotor.
 12. A duct assembly according to claim 8 wherein saidfree rotor and drive rotor are adapted to rotate in opposite directionsin response to said fluid stream.